Move nat-related code to target rtc_base_tests_utils.
Move nat-related code to target rtc_base_tests_utils.

BUG=None

Review-Url: https://codereview.webrtc.org/2591733002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15728}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 01c715096f9e7aa3701ed3893e3527b587af0a89

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk